The known devices employed at present for this purpose comprise generally, between the upstream transmission fiber and the downstream transmission fiber, an optical system intended to make parallel the light rays produced by the upstream fiber, and an optical system focusing the parallel beam anew on the entry of the downstream fiber. In the part where the rays are parallel, the device comprises a semi-reflecting system which withdraws a controlled fraction of the light towards a detector or any other receiver system.
Such a device has the disadvantage, when it is intended to reintroduce a new signal, that the semi-reflecting surface allows the passage of a part of the reintroduced light, which then passes directly towards the detector and falsifies the information which is collected thereat.
French patent application No. 81/08,636 published under the No. 2,505,056 of the same Applicant provides a first solution to this problem by means of a device comprising a main concave spherical mirror whose central part forms another small concave spherical deflecting mirror, with the same apex and the same radius as the main mirror, but whose center C.sub.2 is slightly offset relative to the center C.sub.1 of the main mirror. The main mirror therefore ensures the continuity of transmission of the greater part of the optical signal between an upstream fiber and a downstream fiber which are arranged symmetrically relative to C.sub.1, while the small central deflecting mirror permits a part of the signal to be withdrawn towards an extraction fiber arranged symmetrically to the upstream fiber relative to C.sub.2, and to introduce a new signal through a reinjection fiber arranged symmetrically to the downstream fiber relative to C.sub.2.
However, the manufacture of such a complex mirror, with a small central part whose axis is diverted relative to the main axis, presents technical and economic problems and does not lend itself well to large scale industrial manufacture.